5.0 incompatible changes plan

Nobody has claimed this yet.

- [ ] CHANGELOG for all the new merged stuff since 4.x.
- [x] changed `under_score` algorithm https://github.com/abonas/kubeclient/pull/355
- [x] document watch #436
- [ ] change watch exceptions behavior https://github.com/abonas/kubeclient/pull/275#issuecomment-591928097
- [x] support ruby 3.0 #500
- [ ] drop EOL ruby versions ≤2.4
- [ ] I'm thinking about making all methods not documented in README `private`?
- Specifically `get_entities`, `patch_entity` etc?
These _are_ used in the wild a bit. Should I keep them until alternative #332? But `.public_send("get_#{...}", ...)` while clumsy _is_ workable, `.send` and a major release is perfect time to make people aware they're using undocumented functionality.
- [x] `.rest_client` too. UPDATE: replaced by `.faraday_client`, intentionally public.
- [ ] TODO document
- header manipulation hacks like samson did for #358 ?

I don't want these to hold up 5.0, or block people from migrating.
OTOH, private methods are still available with `.send(...)`, just people will be more aware it's unsupported.

- [x] several tests disabled due to expired TLS certs #483
- [x] truffleruby failing on master with "ArgumentError: unknown keyword: status"
e.g. https://github.com/abonas/kubeclient/pull/495/checks?check_run_id=2044157037
UPDATE: now passes e.g. https://github.com/abonas/kubeclient/runs/2535621628 :man_shrugging:

Review the unchecked checklist items and their linked pull requests, issue comments, and migration concern in this issue. Resolve which API and Ruby-version changes belong in 5.0, update the CHANGELOG, and document the final migration expectations; completion requires the remaining checklist decisions and work to be settled.

Written by the indexing model from the issue text.